Process for preparing organopolysiloxane gum

Claim 1

In a process for preparing an organopolysiloxane gum by effecting condensation polymerization of a silanol-terminated organosiloxane of the following formula (1): ##STR12## wherein R 1 and R 2 each are a halogen- or cyano-substituted or unsubstituted monovalent hydrocarbon group and letter n is an integer of 2 to 3,000, the improvement comprising adding a silane or siloxane having two hydrolyzable groups in a molecule to said orgnopolysiloxane of formula (1) in such an amount to give a resultant composition having a molar ratio of the silanol groups in said organosiloxane of formula (1) to the hydrolyzable groups in said silane or siloxane of from 0.5 to 3, and condensation polymerizing the resultant composition to form an organopolysiloxane having a high degree of polymerization of at least 1×10 3 and a total content of low-molecular weight cyclic siloxanes having from three to 25 monomer units of up to 5,000 parts by weight per million parts by weight of the organopolysiloxane, wherein the silane or siloxane having two hydrolyzable groups in a molecule is selected from the group consisting of the following formulae (3) to (6): ##STR13## wherein R 3 , R 4 , R 5 , and R 6 are each independently a halogen- or cyano-substituted or unsubstituted monovalent hydrocarbon group, and letter a is a positive number of 0 to 100; ##STR14## wherein R 3 , R 4 , R 5 and R 6 are each independently a halogen- or cyano-substituted or unsubstituted monovalent hydrocarbon group and letter c is a positive number of 1 to 100; ##STR15## wherein R 3 , R 4 , R 5 , R 8 and R 9 are each independently a halogen- or cyano-substituted or unsubstituted monovalent hydrocarbon group arid letter d is an integer of 2 to 10; and ##STR16## wherein R 3 , R 4 , and R 5 are each independently a halogen- or cyano-substituted or unsubstituted monovalent hydrocarbon group and letter e is a positive number of 0 to 1,000.